1. Field of the Invention
The present invention relates to a stepping motor, and specifically to a low-profile stepping motor having its dimension reduced in an axial direction.
2. Description of the Related Art
Recently, in accordance with enhanced performance of various devices such as printers, facsimiles and floppy (Registered Trademark) disk drives, a stepping motor is required to be higher in torque, and lower in cogging torque and vibration. Under the circumstance, a PM (permanent magnet) stepping motor has been extensively used to meet the requirements.
FIG. 1 is a partly cutaway perspective view of a conventional PM stepping motor. Referring to FIG. 1, the PM stepping motor generally comprises: a stator assembly 6 composed of two stator units 6A and 6B; a rotor assembly 13; a face plate 1 punched out of a stainless steel plate; and a front bearing 2 made of oil impregnated alloy. The stator units 6A and 6B each comprise: stator yokes 3a and 3b punched out of a soft magnetic steel plate and each having a plurality of pole teeth 10 bent up along its inner circumference; and a coil 4 formed such that a copper wire with polyurethane insulation is wound on a bobbin 5 housed in the stator yokes 3a and 3b, and are attached back-to-back to each other. The rotor assembly 13 comprises: a rotary shaft 8; a cylindrical magnet 9 for magnetic field, having a plurality of magnetic poles at its circumferential surface, and having its circumferential surface opposing the pole teeth 10 of the stator yokes 3a, 3b; and a sleeve 12 to fasten the cylindrical magnet 9 and the rotary shaft 8 together such that the rotary shaft 8 is press fitted into the sleeve 12, and the sleeve 12 with adhesive applied thereon is inserted into the cylindrical magnet 9. The rotor assembly 13 thus structured has its outer circumference (of the magnet 9) magnetized for a predetermined number of magnetic poles, and is rotatably disposed inside the stator assembly 6 such that the rotary shaft 8 is supported by the front bearing 2 and a rear bearing (not shown in FIG. 1), and that the magnetic poles oppose the pole teeth 10 with a slight gap therebetween.
FIG. 2 is a partly cross-sectioned view of the stator unit 6A of the conventional PM stepping motor shown in FIG. 1. The stator unit 6B attached to the stator unit 6A is of the same structure as the stator unit 6A and is omitted. The stator units 6A and 6B are each structured such that the pole teeth 10 of the stator yoke 3a mesh with the pole teeth 10 of the stator yoke 3b equidistantly from each other with a shift by an electrical angle of 180 degrees (half pitch). The stator units 6A and 6B thus structured are joined back-to-back to each other by molding with polymeric material or plasma welding such that their respective pole teeth 10 and 10 are shifted from each other by an electrical angle of 90 degrees, whereby the rotor assembly 13 (shown in FIG. 1) always starts its rotation in the same direction.
FIG. 3 is a developed view of the stator yokes 3a and 3b of the conventional PM stepping motor. In the embodiment the stator units 6A and 6B each have ten pole teeth 10 in a spatial angle of 360 degrees, which are arrayed at an interval corresponding to that of the magnetic poles (not shown) formed on the cylindrical magnet 9.
Recently, since the devices incorporating PM stepping motors are required to be downsized and lower profiled as described above, the PM stepping motors are also required to be lower profiled. The conventional PM stepping motor, however, is composed of two stator units joined to each other back-to-back therefore making it extremely difficult or almost impossible to further reduce its profile. And the conventional PM stepping motor uses four discrete stator yokes, which inevitably increases the number of the components and makes alignment troublesome.
The present invention has been made in view of the above circumstance, and it is an object of the present invention to provide a low-profile stepping motor which is of a simple structure and which has its dimension drastically reduced in the axial direction.
In order to achieve the object, according to a first aspect of the present invention, a low-profile stepping motor generally comprises a first stator unit, a second stator unit, and a rotor assembly including a cylindrical magnet. The first stator unit includes: a first upper stator yoke having a plurality of pole teeth which are formed along its semicircular inner circumference; a first lower stator yoke having a plurality of pole teeth which are formed along its semicircular inner circumference and which mesh with the pole teeth of the first upper stator yoke with a shift by an electrical angle of 180 degrees; and a first coil unit fixedly sandwiched between the first upper and lower stator yokes. The second stator unit includes: a second upper stator yoke formed as one piece integrally with the first upper stator yoke, and having a plurality of pole teeth which are formed along its semicircular inner circumference; a second lower stator yoke formed as one piece integrally with the first lower stator yoke, and having a plurality of pole teeth which are formed along its semicircular inner circumference and which mesh with the pole teeth of the second upper stator yoke with a shift by an electrical angle of 180 degrees; and a second coil unit fixedly sandwiched between the second upper and lower stator yokes and arranged horizontally flush with the first coil unit. Any two pole teeth adjacent to each other with one formed on the first upper stator yoke and the other formed on the second upper stator yoke are shifted from each other by an electrical angle of either 90 degrees or 270 degrees, and any two pole teeth adjacent to each other with one formed on the first lower stator yoke and the other formed on the second lower stator yoke are shifted from each other by an electrical angle of either 90 degrees or 270 degrees. The rotor assembly is rotatably disposed in a circular open space defined by the pole teeth of the first and second stator units. With this structure, the low-profile stepping motor has its axial dimension almost halved without sacrificing its performance, and reduces the number of components thereby achieving cost reduction.
According to a second aspect of the present invention, in the low-profile stepping motor of the first aspect, the first upper and lower stator yokes are fixedly put together by means of a stud such that the stud has its top end squeezed into a hole formed at a predetermined place of the first upper stator yoke and has its bottom end squeezed into a hole formed at a predetermined place of the first lower stator yoke, and the second upper and lower stator yokes are fixedly put together by means of a stud such that the stud has its top end squeezed into a hole formed at a predetermined place of the second upper stator yoke and has its bottom end squeezed into a hole formed at a predetermined place of the second lower stator yoke. With this structure, the stator yokes can be easily and surely aligned to each other thereby reducing cost.
According to a third aspect of the present invention, in the low-profile stepping motor of the second aspect, the coil unit is structured such that a winding is provided around the stud. Accordingly, the coil unit can be easily mounted thereby reducing cost.
According to a fourth aspect of the present invention, in the low-profile stepping motor of the first aspect, a pair of slot cuts are provided at a midway portion between the first and second upper stator yokes, and a pair of slot cuts are provided at a midway portion between the first and second lower stator yokes. This structure keeps the first and second stator units from having respective magnetic influence on each other thereby achieving a smooth rotation.
According to a fifth aspect of the present invention, in the low-profile stepping motor of the fourth aspect, the slot cuts each have a length measuring at least two-thirds of the width of the midway portion between the first and second stator yokes. When the slot cuts are arranged to have their lengths equal to two-thirds of the width of the midway portion, the magnetic influence can be surely minimized while an adequate strength can be maintained at the midway portion. And the lengths of the slot cuts can still be increased if so required, as long as the midway portion retains a minimum strength required.
According to a sixth aspect of the present invention, in the low-profile stepping motor of the first aspect, the pole teeth taper off toward their distal ends so as to be shaped trapezoidal. This configuration of the pole teeth ensures an accurate assembly of the first and second upper stator yokes and the first and second lower stator yokes.